Swagger is a helpful build-in UI interface in ASP.NET Core API to help management and execute all API in your project. It looks like Postman but it is smaller and built-in inside your project. You can test your APIs in the browser right now when you started your project.
Read more article from here https://quizdeveloper.com/tips/easy-to-enable-swagger-ui-interface-in-aspdotnet-core-3dot1-api-aid67